1. Field of the invention:
This invention relates to a semiconductor laser array device which can attain a far-field pattern having a single peak and/or which has a deflecting function.
2. Description of the prior art:
Semiconductor laser devices which are useful as light sources of laser discs, space communication systems, etc., must produce high-output power. However, conventional semiconductor laser devices having a single lasing filament structure can only produce a low 40 mW output power. Even the most advanced semiconductor laser devices would produce 60-70 mW at their best so long as a single filament structure is adopted thereto. Semiconductor laser array devices, in which a plurality of lasing filaments are disposed in a parallel manner to achieve an optical phase coupling between the adjacent filaments, have been proposed by, for example, W. Streifer et al., of Xerox Company, Appl. Phys. Lett. 42,645 (1983), which reports that output power of as high as 1.5 W can be obtained by a gain guided semiconductor laser array device having forty filaments. However, the optical phase shift between the adjacent filaments of this device is 180.degree., resulting in a far-field pattern having two peaks. Moreover, an index guided semiconductor laser array device proposed by D. Botez et al., of RCA Laboratories, Fourth International Conference on Integrated Optics and Optical Fiber Communication, Abstract 29B5-2, Jun. 27-30, 1983, Tokyo, Japan, has a structure of nine filaments which can produce an output power of 200 mW, but this device cannot attain a 0.degree.-phase shift between the adjacent filaments at all, as well.
FIGS. 9(a) and 9(b) show a typical conventional index guided semiconductor laser array device, which can be produced as follows: On an n-GaAs substrate 1, an n-Al.sub.x Ga.sub.1-x As cladding layer 2, an n-(or p-)Al.sub.y Ga.sub.1-y As active layer 3, a p-Al.sub.x Ga.sub.1-x As cladding layer 4 and p.sup.+ -GaAs cap layer 5 are successively grown by liquid phase epitaxy, molecular beam epitaxy, metal organic-chemical vapor deposition, vapor phase epitaxy or the like. Then, a plurality of channels having a width of 2 .mu.m and a pitch of 5 .mu.m, which reaches the inside of the p-cladding layer 4, are formed in a parallel manner by photolithography and an etching technique. On the resulting channeled substrate, an n-Al.sub.z Ga.sub.1-z As buried layer 6 is grown by liquid phase epitaxy to fill the channels therewith (wherein 0.ltoreq.z&lt;y&lt;x or 0.ltoreq.y&lt;x&lt;z). Then, an electrode 8 is formed on the whole back face of the substrate, and an electrode 7 is formed on the upper face of the grown layers in such a manner that it is symmetrical with respect to a line 100 which is parallel to the waveguides (i.e., channels). Finally, faces which are at right angles to the waveguides are formed by a cleavage, resulting in the facets for laser resonation. The resulting device contains the active layer 3 composed of portions 9 having a high equivalent index and the other portions 10 having a low equivalent index. The high effective refractive portions 10 constitute lasing filaments because each of them is constricted by the low effective refractive portions 9.
FIG. 10 shows a typical far-field pattern of the above-mentioned device shown in FIGS. 9(a) and 9(b), exhibiting two peaks with the same optical intensity. The resulting laser which emits light in two different directions cannot be concentrated into a spot fashion by means of any optical lens. Thus, a device attaining such a far-field pattern with two peaks cannot be put into practical use.
The reason why lasers emitting light in two different directions are produced in the conventional semiconductor laser array device can be explained as follows:
FIG. 11 shows an illustration of a laser oscillation process by the device shown in FIGS. 9(a) and 9(b), wherein the waveguides can be taken for lateral gratings 105 because periodic differences .DELTA.n in the refractive index have arisen between the high effective refractive portions 9 and the low effective refractive portions 10 in the active layer 3. The gratings 105 laterally exist with a period of .LAMBDA.. The upper and lower faces of the device have been cleaved to form facets. In the index guided laser array device (i.e., lateral distributed-feedback device), the relationship between the period .LAMBDA. of the gratings 105 and the oscillation wavelength .lambda. within the device can be represented by the equation (1): EQU .+-..lambda./2.LAMBDA.=sin.theta. (1)
wherein .theta. is the angle of the proceeding direction of lightwave to the direction of each of the waveguides.
The lightwave is propagated at the angle of .theta. to the direction of the gratings 105, resulting in two different waves 101 and 102 which proceed in the reverse direction as shown in FIG. 11. The two different lightwaves are radiated from the cleaved facet to produce two different lightwaves 103 and 104, both of which are at angles of .alpha. to the direction at right angles with the cleaved facet. The angle .alpha. is represented by the equation (2): EQU 2.alpha.=.+-.2sin.sup.-1 .lambda..sub.0 /2.LAMBDA. (2)
wherein .lambda..sub.0 is the light wavelength in the air.
On the other hand, deflectable semiconductor laser array devices which can deflect light beams produced therefrom are required for the development of optical integrating circuits in such fields as in optical switches, optical information processing, etc.
Conventional deflecting means utilizing rotatable polygonal mirrors have disadvantages that they cannot be miniaturized and their deflection rate is extremely low, although their deflecting angles are great. Other conventional deflecting means utilizing holograms are disadvantageous in that their optical systems are so complicated that they cannot be miniaturized. Other conventional deflecting means containing dielectric substances utilizing specific electrooptic and/or acoustooptic effects are disadvantageous in that their deflecting angles are extremely small and the driving voltage must be maintained at a high level. Moreover, since the above-mentioned conventional deflecting means have no light-emitting function, they must be optically and precisely coupled to gas lasers or semiconductor lasers serving as light sources for practical use. Such optical and precise coupling of the deflecting means to the light sources is too troublesome for practical use.